1. Field of the Invention
The present invention relates to a membrane cartridge and a method for producing a membrane cartridge.
2. Description of the Related Art
An immersion-type membrane separation device is suitably used as a solid-liquid separation device in a facility for water treatment such as wastewater purification treatment. The immersion-type membrane separation device contains a plurality of flat-plate membrane cartridges each of which includes a filtration plate made of a resin and a filtration membrane provided on both sides of the filtration plate and joined thereto at a periphery thereof.
Japanese Patent Publication No. H06-218240 discloses a method for producing a membrane module aiming at preventing bonding unevenness of a filtration membrane to a filtration plate, improving bonding efficiency, and thereby securing the airtightness of the membrane module, by covering a surface of an organic material membrane support body with an organic filtration membrane, and thermally bonding the periphery of the organic filtration membrane to the membrane supporting body by local heating.
Japanese Patent Publication No. 2001-120958 proposes an immersion-type membrane cartridge production method in order to provide a producing method for an immersion-type membrane cartridge having an improved durability against aeration.
The production method of the immersion-type membrane cartridge includes forming on a filtration plate made of a resin double-lined sealing portions and a band-like auxiliary portion which protrude from a surface of the filtration plate and are integrally formed along the periphery of the filtration plate to make a perimeter, wherein an inside sealing portion has a profile lower than that of an outside sealing portion, and the outside sealing portion and the auxiliary portion have the same height. The method further includes disposing a filtration membrane over the surface of the filtration plate so as to cover both of the sealing portions and the auxiliary portion, pressing an up-down horn onto the sealing portions and the auxiliary portion over the filtration membrane, and emitting ultrasonic waves so as to weld the filtration membrane at the sealing portions and the auxiliary portion, thereby forming a liner water stopping portion at the sealing portions so as to maintain the filtration membrane in a tension state, while forming an auxiliary welding portion at the auxiliary portion.
Japanese Patent Publication No. 2006-231139 discloses a membrane cartridge including a support body of a non-woven fabric made of plastic fiber and a microporous filtration membrane bonded onto a smooth surface in a periphery of a filtration plate made of a thermoplastic resin, intending to obtain a membrane cartridge capable of replacing the filtration membrane by reusing the thermoplastic filtration plate when the membrane has broken or deteriorated.
In all of the membrane cartridges disclosed in Japanese Patent Publication Nos. H06-218240, 2001-120958, and 2006-231139, the bonding surface of the filtration membrane onto the filtration plate is a plane parallel to the surface of the filtration plate.
It is generally recognized that, in a plate-like membrane cartridge, a filtration membrane is clinging to a filtration plate by a suction pressure or a natural water head pressure during a filtration operation, is trembling in a slightly bulged state during a relaxation in which only air diffusion is performed without filtration operation, and is kept in a bulged state during a clearing operation under a reversed pressure in which a chemical solution or clear water is injected from the secondary side of the filtration membrane.
In actuality however, the suction pressure during the filtration operation is rather small and in a range of 10 kPa, and the membrane cartridge vibrates vigorously due to an upward flow accompanying the air diffusion from below. Accordingly, the filtration membrane may not perfectly follow the vibration of the filtration plate, but is repeatedly separating from and coming into contact with the filtration plate.
When the filtration membrane repeatedly bulges from and clings to the filtration plate, a joining portion of the filtration membrane to the filtration plate may be fatigued, problematically causing a membrane rapture or membrane detachment.
Accordingly, in order to avoid such a membrane rapture or membrane detachment due to fatigue, Japanese Patent Publication No. 2001-120958 described above discloses a filtration plate structure in which the double-lined sealing portions and the band-like auxiliary portion are formed integrally on the filtration plate along the periphery thereof so as to protrude from the surface of the filtration plate to make the perimeter thereof, where the inside sealing portion has the profile lower than that of the outside sealing portion.
However, a bonding process of a filtration membrane onto a filtration plate having such a complicated shape increases various costs for producing, leaving room for further improvements to increase durability against vibration while reducing the costs.